As there is no requirement for registered charities to issue receipts at all, there is no requirement in terms of when receipts would be issued but for the average donor it is helpful to issue them by the end of February so that donors can use the receipt for their tax filing.  Here is a note from CRA:

“When should a charity issue a receipt?
There is no requirement in the Income Tax Act for a registered charity to issue an official donation receipt or that it issue a receipt within a certain timeframe.
The Canada Revenue Agency (CRA) suggests that registered charities issue receipts by February 28 of the calendar year that follows the year of the donation. This allows individual taxpayers to claim their donations on their annual income tax returns.
A registered charity may issue receipts periodically through the year or, for cash donations, issue one cumulative receipt for the year.
A separate receipt must be issued for each gift in kind (non-cash) donation.”